U.S. Pat. No. 6,577,266, issued Jun. 10, 2003 to Axline et al entitled “Transponder Data Processing Methods and Systems,” and U.S. Pat. No. 5,486,830, issued Jan. 23, 1996 to Axline entitled “Radar Transponder Apparatus and Signal Processing Technique,” provide background for an in-band tag system and the present invention. These patents are herein incorporated by reference into the current specification for their teachings.
During operation, a tag receives pulses from the radar, modulates and amplifies these pulses, and returns the pulses without changing the pulses' center frequencies. Because echoes from passive terrain also return to the radar at about this same center frequency and at about the same time as the tag's echo, the passive echoes can interfere with the radar's ability to detect the tag and decode data up-linked from the tag. Passive terrain echoes can be referred to as clutter due to the fact that they are not desirable and serve only to reduce our ability to process the desired echoes coming from the tag. Therefore, some method of attenuating or suppressing these clutter echoes is desired.
As described in the '830 patent, one method to suppress clutter is to impart an interpulse (i.e., pulse-to-pulse) phase code to the tag's echo. Subsequent ‘azimuth’ or ‘Doppler’ processing in the radar can then preferentially enhance the tag's signature by reducing the amount of clutter power present in the processed signal. This clutter suppression can be of the order of a factor of 10 dB or more, its size depending upon the design details of the radar, its antenna, and the velocity at which the radar platform moves as the radar transmits and collects echo pulses from the terrain and from the tag or tags in the illuminated scene. In order to accomplish the suppression of clutter as described in the '830 patent, the radar must process, or integrate, many tag echo pulses. That is, the aforementioned method does not achieve any clutter suppression on a single pulse.
The '830 patent teaches in detail how an airborne synthetic aperture radar (SAR) can command, image, geolocate, and receive uplinked data from a small, remotely located, battery-powered, radio-frequency transponder (herein referred to as a tag). A radar system can interact with the remote tag in either SAR or moving-target-indicator (MTI) modes of the radar. As described in the referenced patent, the passive clutter echo in the imaged scene can be suppressed through use of special pulse-to-pulse (or interpulse) modulation imparted by the tag to its retransmitted pulse and through use of special processing in the radar. This suppression of passive clutter enhances the signature of the tag in the pseudo-image of the tag formed by special processing in the radar.
The aforementioned method of clutter suppression, which is referred to as Doppler processing, or azimuth processing, can be effective in reducing clutter by 10 dB or more. That same method requires that the radar process (or integrate) many tag echo pulses in order to achieve the desired suppression. The exact amount of clutter suppression achieved depends upon the details of the radar-tag system design. The present inventors recognize that a need exists for a more robust clutter reduction process that is not so heavily dependent on integration of tag echo pulses.